The present invention relates to an air-fuel mixture heating device for use in internal combustion engines.
In general, the operation of an internal combustion engine before the warming up and hence at low engine temperature involves a problem in that the fuel supplied by the carburetor is not satisfactorily evaporated so that a large quantity of fuel in liquid state is supplied to the engine such as to worsen the combustion, resulting in an unstable state of engine operation as compared with that after the warming up. In order to obviate this problem, it has been proposed to supply richer air-fuel mixture to the engine during the warming up than in the period after the warming up, thereby stabilizing the engine operation during the warming up. The supply of rich mixture, however, not only increases noxious components such as unburnt hydrocarbon (HC) and carbon monoxide (CO) but increases the fuel consumption uneconomically.
If the fuel in liquid phase supplied from the carburetor during the warming up of the engine is sufficiently evaporated into gaseous phase, the engine will be able to operate stably even with a lean mixture. Such a lean mixture in turn will decrease the noxious exhaust emissions, as well as the fuel consumption.
From this point of view, the specification of U.S. Pat. No. 4,141,327 proposes an intake heating device which is designed for promoting the evaporation of liquid fuel during warming up of the engine.
This intake heating device, however, suffers from the following disadvantages. Firstly, it is to be pointed out that the PTC (Positive Temperature Coefficient of Resistivity) element, which serves as a heat generator constituting an essential part of this device, requires quite a complicated assembly process and raises the production cost because of its honeycomb type structure.
Secondly, the honeycomb type structure which has large wall thickness inevitably has a large outer dimensions in order to obtain a required total cross-sectional area of passages. It is rather difficult to find the room for accommodating such large-sized structure.
Thirdly, small passages for mixtures defined in the honeycomb type PTC heat generator tend to be blocked due to freezing of water content of the air in the mixture, particularly when the engine is used in the atmosphere of low temperature and high humidity, resulting in an engine stall due to shortage of the air-fuel mixture.